Solid state UV laser

ABSTRACT

An apparatus ( 10   a ) for scanning a light beam ( 16 ) through a crystal arrangement ( 22 ) that has a first frame member ( 28 ), a first support member ( 24 ) to support a first optical element ( 18 ) in the first frame member ( 28 ) and a drive mechanism ( 32 ) to rotate the first support member ( 24 ). The first support member ( 24 ) is rotatably mounted in the first frame member ( 28 ). The first optical element ( 18 ) is arranged to receive the light beam ( 16 ). The first optical element ( 18 ) is supported by the first support member ( 24 ) so that the first optical element ( 18 ) is rotatable with the first support member ( 24 ). The first optical element ( 24 ) is tiltable about a diametral line of the first optical element ( 18 ), relative to the axis of the light beam ( 16 ) to be scanned through the crystal arrangement ( 22 ). A frequency conversion apparatus ( 50 ) provides a first crystal set ( 52 ) and a second crystal set ( 54 ). Each of the first and second crystal sets ( 52, 54 ) has at least one non-linear optical crystal. The first crystal set ( 52 ) receives a beam ( 16 ) of coherent radiation that passes through the first crystal set ( 52 ). The second crystal set ( 54 ) receives the beam ( 16 ) after it passes through the first crystal set ( 52 ), and the beam ( 16 ) then passes through the second crystal set ( 54 ). One of the first and second crystal sets ( 52, 54 ) is aligned for frequency conversion of the beam ( 16 ) in the cold state of that crystal set and the other one of the first and second crystal sets ( 52, 54 ) is aligned for frequency conversion of the beam ( 16 ) in the warm steady state of that other crystal set.

FIELD OF THE INVENTION

The present invention relates to solid state laser technology. Thepresent invention finds application, for example, in surgical andmedical fields, including the correction of refractive errors of the eyeby photo refractive keratectomy (PRK) and laser in-situ keratomileusis(LASIK). The present invention also has application in materialprocessing fields, including photolithography, such as the manufactureof microchips, writing defraction gratings in fibre optic cables andglass marking.

BACKGROUND OF THE INVENTION

This specification refers to and describes content of U.S. Pat. Nos.4,346,314, 5,144,630 and 5,592,325 and international patent applicationPCT/AU98/00554 (WO 99/04317). However, neither the disclosures in thoseUS patents and international patent application nor the descriptionscontained herein of those US patents and international patentapplication are to be taken as forming part of the common generalknowledge solely by virtue of the inclusion herein of the reference toand description of content that those U.S. patents and internationalpatent application. Furthermore, this specification describes aspects ofprior art lasers. However, neither such aspects of prior art lasers northe description contained herein of such aspects of prior art lasers isto be taken as forming part of the common general knowledge solely byvirtue of the inclusion herein of reference to and description of suchaspects of prior art lasers.

Excimer gas lasers with an operating wavelength of 193 nm in theultraviolet (UV) region of the electromagnetic spectrum have beenutilised in many of the above applications. The short UV wavelength ofthese lasers processes material through photoablation. The materialbeing processed is vaporized by the laser but little thermal damage iscaused to adjacent areas. This has led to the widespread use of excimerlasers in the medical field. However, excimer lasers do havedisadvantages. These disadvantages include poor reliability, highoperating costs, and the need to use an extremely toxic gas. The gasalso has a limited lifetime in the laser cavity and so must be replacedfrequently. This adds the difficulties associated with handling andshipping a dangerous gas to the excimer laser disadvantages.

On the other hand, solid-state lasers are smaller, more reliable andeasier to use than gas excimer lasers. These lasers utilize glass orcrystal matrices, such as yttrium aluminium garnet (YAG), yttriumlithium fluoride (YLF) or potassium gadolinium tungstate (KGW) that aredoped with rare-earth elements such as neodymium (Nd), erbium (Er) orholmium (Ho). Solid-state lasers are identified by the element and glassor crystal used. For example, a laser using a YAG crystal doped withneodymium is denoted Nd:YAG. This material is referred to as the lasermedium. Excitation of the laser medium, usually by either flash lamp ordiode lasers, results in high-energy laser emissions. These high-energylaser emissions have a variety of wavelengths. The rare-earth element inthe laser medium determines the wavelengths that are produced. However,none of these solid state lasers produce laser emissions that are in theUV wavelength range of the laser emissions produced by excimer lasers.Some of the more common solid state lasers and the wavelengths of theirlaser emissions are Nd:KGW at 1.067 microns, Nd:YAG at 1.064 microns,Nd:YLF at 1.053 microns, Ho:YAG at 2.1 microns and Er:YAG at 2.94microns. These are all in the infra-red portion of the electromagneticspectrum, i.e. they have a (relatively) much longer wavelength than thatof gas excimer lasers.

Whilst solid state lasers produce beams having longer wavelengths thanthose of gas excimer laser, they have been successfully applied todifferent medical and industrial processes. Even so, the longerinfra-red wavelengths produced by solid state lasers makes themunsuitable for most of the applications using excimer lasers.Furthermore, they may produce undesirable effects when applied to somematerials, such as corneal tissue.

It is possible to use non-linear optical (NLO) crystals to convert theinfra-red wavelengths produced by solid state lasers, to shorter visibleand ultraviolet wavelengths. U.S. Pat. No. 5,144,630 describes the useof non-linear optical crystals for frequency conversion of highintensity laser emission. This property of NLO crystals means thatpassage of a laser beam through such a crystal can result in thewavelength of the beam being altered. This property enables the laserbeam produced by an infra-red laser, such as Nd:YAG at 1064 nm, to beconverted to a shorter wavelength of 532 nm. This process is known asharmonic generation (and is described in U.S. Pat. No. 5,592,325 andU.S. Pat. No. 4,346,314). Converting an original infra-red laser beam,at 1064 nm, to a beam with a wavelength at 532 nm is known as secondharmonic generator (SHG). The ability to generate higher harmonics, suchas the fourth and fifth harmonic wavelengths of a Nd:YAG laser, at 266nm and 213 nm, respectively, means that the solid state laser becomessuitable for further applications.

There is a wide range of non-linear optical crystals that can be usedfor harmonic generation to shorter wavelengths. Examples are crystals ofthe borate family, and include beta barium borate (β-BaB₂O₄ or BBO),lithium borate (LBO), caesium lithium borate (CLBO), MBeBo₃F₂ andCsB₃O₅. Other examples of NLO crystals for harmonic generation includePotassium Titanyl Phosphate (KTP or KTiOPO₄) and potassium DideuteriumPhosphate (KD*P or KD₂PO₄) (as described in U.S. Pat. No. 5,144,630 andU.S. Pat. No. 5,592,325).

For the harmonic generation process to work properly, the laser beammust pass through the non-linear crystal at exactly the right anglerelative to the crystal structure. A very small error in the angle thatthe laser beam passes through the crystal can cause the conversionefficiency to drop significantly, possibly even to zero. Fundamentalproblems exist with non-linear optical crystals. Firstly, the exactrequired angle through the crystal usually depends on the temperature ofthe crystal and temperature gradients within the crystal. Secondly, thecrystal usually absorbs a little of either or both the incident longerwavelength and the newly generated harmonic shorter wavelength. Thisabsorbed laser energy heats the crystal, changing its temperature andcreating temperature gradients within the crystal. Thus, the requiredangle through the crystal for efficient harmonic generation when thecrystal is cold, i.e. at the time the laser has just been switched on,is different from the required angle when the laser has been running fora while and its heating of the crystal has reached a steady state. Whena laser is first switched on and the laser beam passes through thecrystal at the angle required for warm steady state efficient harmonicgeneration, it is not unusual for there to be no harmonic generation atall. In such an instance, the harmonic wavelength cannot contribute toheating of the crystal, and therefore the temperature state of thecrystal that produces any harmonic generation is never reached. Evenwhen the differences in angles between the cold starting condition andthe warm steady state condition are not sufficient enough to create theproblem described above, the changes in optimum angle do create longwarm-up times and potentially large swings in the energy of thegenerated harmonic wavelength. To reach the fourth or fifth harmonic,for example 266 nm or 213 nm for Nd:YAG, the conversion process usuallyrequires two or three crystal stages respectively. The instabilities ofenergy are thus multiplied for these shorter wavelengths. Thereforethese solid state UV wavelength laser sources have generally beenconsidered unsuitable for industrial or medical applications.

One proposed solution was to keep the laser pulse repetition rate low toallow the crystal to cool and partially return to its initial statebetween pulses (as described in international applicationPCT/AU98/00554). However, in many industrial applications the low pulserepetition rate makes the application uneconomic due to slow materialprocessing rates. Even in the medical applications of laser refractivesurgery, the low pulse repetition rate can lead to impractically longtreatment times. This is particularly true in the newer types oftreatments based on topography or wave front linked customized ablationsthat require many smaller pulses to be applied to the cornea.

Thus, solid state UV lasers still have undesirable instability issues.With improvements in diode lasers in recent times there is now apreference that solid state lasers are diode laser pumped instead offlash-lamp pumped. Diode laser pumped solid state lasers are potentiallymore reliable and have better energy stability in their infra-red laseroutput than flash-lamp pumped solid state lasers. However, diode laserpumped systems are extremely inefficient at the low pulse repetitionrates proposed in the solution mentioned above. Therefore, diode laserpumped solid state lasers, in particular, need a better solution to theinstabilities of generating UV wavelengths through non-linear opticalfrequency conversions.

DISCLOSURE OF THE INVENTION

In accordance with a first aspect of the present invention there isprovided an apparatus for scanning a light beam through a crystalarrangement comprising:

-   -   a first frame member,    -   a second frame member spaced from said first frame member by a        distance sufficient to accommodate the crystal arrangement,    -   a first support member to support a first optical element in        said first frame member so that said first optical element is        tiltable about a diametral line of said first optical element,        said first support member being rotatably mounted in said first        frame member and said first optical element being arranged to        receive said light beam,    -   a second support member to support a second optical element in        said second frame member so that said second optical element is        tiltable about a diametral line of said second optical element,        said second support member being rotatably mounted in said        second frame member and said second optical element being        arranged to receive said light beam after passage through said        crystal arrangement, and    -   drive means to rotate said first and second support members and        thereby said first and second optical elements while tilted, in        phase, whereby said light beam is scanned over said crystal        arrangement and its path through the crystal arrangement changes        while its orientation does not change, which drive means        includes a rotatable shaft transversely spaced from the axis of        said light beam and rotationally drivingly coupled to said first        and second support members.

Preferably, passage of the light beam through said first optical elementresults in the path of said light beam, after passage through said firstoptical element, being deviated from its path prior to passage throughsaid first optical element, and rotation of said first optical elementresults in the deviated beam spatially moving over time such that thepath of the deviated beam through said crystal arrangement changes tothereby scan said light beam through said crystal arrangement.

A beam scanning device may be provided on the beam output side of thecrystal arrangement.

Preferably, the scanned light beam output from said crystal arrangementis directed to said second optical element to return said deviated beamto a beam that does not spatially move over time to thereby descan saidlight beam.

Preferably, each of the first and second optical elements is tiltable atan angle to the axis of the light beam and is rotatable about an axisthat is substantially parallel to the axis of said light beam.

Preferably, the first and second optical elements are tiltable inopposite directions relative to the axis of said light beam.

Preferably, said drive means comprises a motor and a drive shaftextending therefrom, a first wheel mounted on said drive shaft and asecond wheel connected with said first support member and belt meansextending between first and second wheels to transfer rotational drivefrom said drive shaft to said first support member via said first andsecond wheels and said belt means to thereby rotate said first supportmember and said first optical element.

Preferably, said drive shaft extends from said motor toward said secondframe member and is transversely spaced from the axis of said lightbeam, a third wheel is mounted on said drive shaft, a fourth wheel isconnected with said second support member and second belt means extendsbetween said third wheel and said fourth wheel such that rotationaldrive of said drive shaft is transferable to said second support membervia said third and fourth wheels and said second belt means to therebyrotate said second support member and said second optical element.

Preferably, said drive means is supported by bracket means connected toat least said first frame member.

Preferably, said bracket means comprises a first bracket and a secondbracket connected to said first frame member and said second framemember, respectively, said first bracket supporting said motor and saidfirst wheel and said second racket supporting said third wheel and thedistal end of said drive shaft.

The apparatus for scanning a light beam through a crystal arrangement ashereinbefore described may be incorporated into a laser apparatus whichalso comprises a solid state laser to emit a beam of coherent radiation.Accordingly, in accordance with a second aspect of the present inventionthere is provided a solid state laser apparatus comprising:

-   -   a apparatus for scanning a light beam through a crystal        arrangement as hereinbefore described, and    -   a solid state laser to emit a laser beam, which forms the light        beam that is scanned by the apparatus for scanning a light beam        through a crystal arrangement.

The apparatus for scanning a light beam through a crystal arrangement aspreviously hereinbefore described may be incorporated into an existingsolid state laser. However, in view of the tolerances that are requiredbetween the solid state laser and the frequency conversion apparatus,this would normally be a difficult and non cost-effective procedure.

In accordance with a third aspect of the present invention there isprovided a frequency conversion apparatus comprising:

-   -   a first crystal set having at least one non-linear optical        crystal,    -   a second crystal set having at least one non-linear optical        crystal,    -   said first crystal set is arranged to receive a beam of coherent        radiation that passes through said first crystal set,    -   said second crystal set is arranged to receive said beam after        passage through said first crystal set, and said beam passes        through said second crystal set,    -   wherein one of said first and second crystal sets is aligned for        frequency conversion of said beam in the cold state of that        crystal set and the other of said first and second crystal sets        is aligned for frequency conversion of said beam in the warm        steady state of said other crystal set.

The frequency conversion apparatus in accordance with the third aspectof the present invention may be incorporated into a solid state laserapparatus that further includes a solid state laser.

Preferably, the first and second crystal sets are aligned for frequencyconversion for said cold state or said warm steady state by adjustingthe physical orientation of a respective said crystal in said first andsecond crystal sets.

Alternatively, or in addition, the first and second crystal sets may bealigned for frequency conversion in said cold state or said warm steadystate by altering the temperature of a respective said crystal in saidfirst and second crystal sets to adjust the crystal structure of saidrespective crystal.

Preferably, said first crystal set is aligned for frequency conversionin the cold state and said second crystal set is aligned for frequencyconversion in the warm steady state.

In an alternative embodiment, the frequency conversion apparatus inaccordance with the third aspect of the present invention furthercomprises beam splitter means to divert a component beam of shortestwavelength of the output beam from said first crystal set away from theremainder of the output beam from said first crystal set and said beamsplitter means allows the remainder of the output beam from said firstcrystal set to pass to said second crystal set for frequency conversiontherein.

In a further embodiment, the frequency conversion apparatus inaccordance with the third aspect of the present invention furthercomprises beam combining means to receive the component beam of shortestwavelength diverted by said beam splitter means and the output beam fromsaid second crystal set and to combine them into a single beam.

Preferably, said beam combining means comprises polarisation optics.

In accordance with a fifth aspect of the present invention there isprovided a method of improving the frequency conversion efficiency of afrequency conversion system, of a laser apparatus, that has a firstcrystal set having at least one non-linear optical crystal, comprising:

-   -   placing a second crystal set having at least one non-linear        optical crystal at the beam output end of said first crystal        set,    -   aligning one of said first and second crystal sets for frequency        conversion of an input beam in the cold state of that crystal        set, and    -   aligning the other of said first and second crystal sets for        frequency conversion of an input beam in the warm steady state        of said other crystal set.

Preferably, aligning of one said first and second crystals sets forfrequency conversion of said beam in the cold state or the warm steadystate comprises adjusting the physical orientation of the respectivecrystal in said first and second crystal sets.

Alternatively, or in addition, aligning said first and second crystalsets for frequency conversion in the cold state or the warm steady statecomprises altering the temperature of the respective said crystal insaid first and second crystal sets to adjust the crystal structure ofsaid respective crystal.

The apparatus for scanning a light beam through a crystal arrangementand the frequency conversion apparatus in accordance with the first andthird aspects of the present invention may be used together and may beincorporated into a single laser apparatus that also comprises a solidstate laser.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will now be described, by way of example, withreference to the accompanying drawings in which:

FIG. 1 a is a perspective view of a portion of an embodiment of a solidstate laser apparatus, incorporating a frequency conversion apparatus,in accordance with a first aspect of the present invention;

FIG. 1 b is a first perspective view of an embodiment of an apparatusfor scanning a light beam through a crystal arrangement, which alsoforms a further portion of the apparatus shown in FIG. 1;

FIG. 1 c is a second (exploded) perspective view of the apparatus shownin FIG. 1 b;

FIGS. 2 a, 2 b, 2 c and 2 d are views showing the two optics of theapparatus shown in FIGS. 1 a and 2 b in four sequential positions;

FIG. 3 shows the non-linear optical crystal of the apparatus shown inFIGS. 1 a illustrating the sequential spatial positions of the beamentering the non-linear optical crystal corresponding to FIGS. 2 a, 2 b,2 c and 2 d;

FIG. 4 is a schematic view of an embodiment of a solid state laserapparatus incorporating a first embodiment of a frequency conversionapparatus in accordance with a further aspect of the present invention;

FIG. 5 is a schematic view of a second embodiment of a solid statuslaser apparatus incorporating a second embodiment of a frequencyconversion apparatus in accordance with the further aspect of thepresent invention;

FIG. 6 is a schematic view of another embodiment of a solid state laserapparatus incorporating a third embodiment of a frequency conversionapparatus in accordance with the third aspect of the present invention;and

FIG. 7 is a schematic view of an embodiment of a solid state laserapparatus incorporating a frequency conversion apparatus of the solidstate laser apparatus illustrated in FIGS. 1 a to 3 and a frequencyconversion apparatus illustrated in FIG. 4.

BEST MODE(S) FOR CARRYING OUT THE INVENTION

In FIGS. 1 a, 1 b and 1 c there is shown an embodiment of a solid statelaser apparatus 10 in accordance with one aspect of the presentinvention. The solid state laser apparatus 10 has been split into FIGS.1 a and 1 b for visual clarity so as not to obscure features of thesolid state laser apparatus 10. Additionally, FIGS. 1 b and 1 c shows anembodiment of an apparatus 10 a for scanning a light beam through acrystal arrangement.

The solid state laser apparatus 10 comprises a solid state laser 12, afrequency conversion apparatus, or system, 14 and the apparatus 10 a.The solid state laser 12 emits an infra-red (IR) beam 16 of coherentradiation. The frequency conversion apparatus 14 subjects the beam 16 tofrequency conversion, i.e. harmonic generation.

The frequency conversion apparatus 14 comprises at least one NLO crystal22, positioned between the first and second optics 18 and 20 of theapparatus 10 a.

Each of the optics 18 and 20 of the apparatus 10 a is supported by asupport 24 and 26, respectively. The supports 24 and 26 are rotatablymounted in respective frame members 28 and 30 by bearings (that areobscured in the drawings). The frame members 28 and 30 have respectiveopenings 28 a and 30 a therein to accommodate the supports 24 and 26.The supports 24 and 26 ring-like in form.

A motor 32 is provided to rotate the supports 24 and 26 in theirrespective frame members 28 and 30. The motor 32 is mounted on a bracket34 which is connected to the frame member 28. A drive shaft 36 extendsfrom the motor 32 between the bracket 34 and another bracket 38 that issupported by the frame member 30.

A first wheel 40 is mounted on the drive shaft 36 adjacent the bracket34. A second wheel 42 is attached to the support 24 that supports theoptic 18 in the frame member 28. Another wheel 40 is provided on thedistal end of the drive shaft adjacent the bracket 38 and another wheel42 is attached to the support 26 that supports the optic 20 in the framemember 30.

A chain or belt 44 extends between each pair of wheels 40 and 42,respectively. The wheels 40 rotate with the drive shaft 36 and the chainor belt 44 transfers rotation to the wheels 42 attached to the supports24 and 26. In this way, the supports 24 and 26, and therefore the optics18 and 20, rotate together in phase.

The drive shaft 36 extends between the brackets 34 and 38 such that itis transversely spaced from the axis of the beam 16.

The crystal 22 is positioned between the frames 28 and 30 and therebybetween the first and second optics 18 and 20.

The optics 18 and 20 each have two flat parallel surfaces 18 a, 18 b and20 a, 20 b, respectively.

The optics 18 and 20 are supported by their respective supports 24 and26 such that they are tiltable, about a respective diametral linethereof, at an angle relative to the axis of the beam 16. To achievethis, each optic 18 and 20 is held in a respective frame 18 a and 20 a.Each frame 18 a and 20 a is pivotally attached to the respectivesupports 24 and 26. This may be done by using a pair of diametriallyopposed pivot pins on each frame 18 a and 20 a. The pivot pins on eachframe 18 a and 20 a are held in corresponding pin end receptors providedin the supports 24 and 26. The optics 18 and 20 are tilted at an anglethat is greater than 0° and less than 90° relative to the axis of thebeam 16.

The optics 18 and 20 are tilted in opposed directions such that they arerotationally 180° out of phase with one another.

The supports 24 and 26 rotate the optics 18 and 20, respectively, aboutan axis that is parallel to the beam 16.

Whilst a motor 32, drive shaft 36, wheels 40 and 42 and a chain or belt44 is used to rotate the supports 24 and 26 and thereby the optics 18and 20, alternative arrangements may be used to rotate the optics 18 and20 provided that the optics 18 and 20 are rotated at the same speed.

In use, the beam 16 emitted by the solid state laser 12 passes throughthe optic 18. Due to the tilt of the optic 18, the path of the beam 16is deviated by the optic 18 such that the beam 16, after it has passedthrough the optic 18, is no longer co-linear with the beam 16 prior topassage through the optic 18. However, the beam 16 after passage throughthe optic 18 is parallel to the beam 16 prior to passage through theoptic 18. Thus, the beam 16 following passage through the optic 18remains parallel to, and therefore at the required angle to pass throughthe crystal 22 for efficient harmonic generation.

Rotation of the optic 18 means that once the beam 16 passes through theoptic 18, it spacially moves. This is illustrated in FIGS. 2 a-2 d andFIG. 3. FIGS. 2 a-2 d show four positions of the optics 18 and 20 in acycle of rotation of the optics 18 and 20. Each of the FIGS. 2 a, 2 b, 2c and 2 d is a sequential view of the position reached by the optics 18and 20 after rotation through a further 90° from the position in thepreceding figure. The direction of rotation of the optics 18 and 20 isshown by the arrow R in FIG. 1 a. The beam 16 after passage through theoptic 18 is identified by reference numerals 16A, 16B, 16C and 16D inFIGS. 2 a, 2 b, 2 c and 2 d, respectively.

FIG. 3 shows the positions of the beams 16A, 16B, 16C and 16D of FIGS. 2a, 2 b, 2 c and 2 d, respectively, when they impinge the surface of thecrystal 22. The rotation of the optic 18 causes the path of the beam 16,after passage through the optic 18, to be spacially moved over time, orscanned, so that its paths form a cylindrical surface. Correspondingly,as the beam 16 moves in this way, it traces a circular pattern 46 on thesurface of the crystal 22. The beam travel through the crystal 22 issimilarly scanned through the crystal 22 so that the path through thecrystal 22 changes and the beam 16 does not travel through the sameregion of the crystal 22.

Moving the beam 16 through the crystal 22 is this way distributes thethermal load from the beam 16 on the crystal 22 over a larger regionthan in the case of a non-scanned laser beam that constantly travelsthrough the same region of the crystal. Distributing, or spreading out,the thermal load on the crystal 22 helps to keep the temperature of thecrystal 22 closer to the initial state and minimises changes intemperature gradients. This maintains better stability of the frequencyconversion performed by the crystal 22.

The shorter wavelength beam 16 s that is output from the crystal 22,following frequency conversion thereby, will also be scanned to define acylindrical surface when it exits the crystal 22. The output beam 16 spasses through the rotating optic 20 which moves the output beam 16 sback onto the original path of the input beam 16 (so that it is againco-linear with the beam 16) and de-scans the output beam 16 s such thatit no longer spatially moves over time, i.e. its spatial positionovertime is constant.

As the output beam 16 s has a wavelength different from that of theinput beam 16, the optic 20 through which the output beam 16 s passesmay need to be of a different material, thickness and/or tilt angle sothat the output beam 16 s is returned to the path of the input beam 16when it exits the optic 20.

In some applications, the optic 20 may not be required and the outputbeam 16s would be de-scanned in another way. For example, if the outputbeam 16 s was to pass through another scanner device that is used aspart of the material processing application on which the apparatus 10 isbeing used, the control of that scanner device could be adjusted tocompensate for the changing position of the output beam 16 s and therebyreturn the output beam 16 s onto the path of the input beam 16.

In another alternative, some applications may not require that theoutput beam 16 s is de-scanned. In such applications, the optic 20 mayagain be omitted and the output beam 16 s would trace a circular patternon the material being processed by the apparatus 10.

The optics 18 and 20 may be made from any suitable material that permitstransmission therethrough of appropriate wavelengths, viz. IRwavelengths for the optic 18 and UV wavelengths for the optic 20. Suchmaterials are known in the art.

Whilst the preceding embodiment has been described with only a singleNLO crystal 22 being used, which will result in the output beam 16 sbeing a second harmonic, it is to be understood that additional crystals22 can be used to obtain an output beam 16 s having higher harmonics,e.g. third, fourth, fifth harmonics.

The crystal 22 may be made of known materials for providing harmonicgeneration of the input beam as previously described herein.

Similarly, the solid state laser 12 may be of known type as previouslydescribed herein.

In FIG. 4 there is shown an embodiment of a laser apparatus 10A inaccordance with another aspect of the present invention. Similarcomponents of the apparatus 10A and the apparatus 10 are denoted bysimilar reference numerals in FIG. 4. The apparatus 10A comprises asolid state laser 12 that is similar to the solid state laser 12 of theapparatus 10. The apparatus 10A further comprises a frequency conversionapparatus 50.

The frequency conversion apparatus 50 comprises a first crystal set 52and a second crystal set 54. The first crystal set 52 comprises NLOcrystals 22 a, 22 b and 22 c. The second crystal set 54 comprises NLOcrystals 22 aa, 22 bb, and 22 cc.

Providing three crystals in the first and second crystal sets 52 and 54results in the fifth harmonic of the input beam 16 of coherentradiation, being created by the frequency conversion performed by thecrystals 22 a, 22 b and 22 c and the crystals 22 aa, 22 bb and 22 cc.However, a different, e.g. lesser, number of crystals may be provided inthe first and second crystal sets 52 and 54 for generation of different,e.g. lesser, harmonics.

The beam 16 is received at the first crystal set 52 and passes throughthe crystals 22 a, 22 b and 22 c of the first crystal set 52, and theoutput beam from the last crystal 22 c, of the first crystal set 52, isidentified by reference numeral 16 a for ease of identification. Theoutput beam 16 a from the last crystal 22 c of the first crystal set 52is received at the second crystal set 54 and passes through the crystals22 aa, 22 bb and 22 cc of the second crystal set 54. The output beamfrom the last crystal 22 cc of the second crystal set 54 is identifiedby reference numeral 16 b for ease of identification.

The first crystal set 52 is aligned for frequency conversion, i.eharmonic generation, in the cold state. That is, the crystals 22 a, 22 band 22 c of the first crystal set 54 are aligned such that the angles ofthe crystal structures of the crystals 22 a, 22 b and 22 c relative tothe beam 16 received at the first crystal set 52, are orientated forefficient harmonic generation when the crystals 22 a, 22 b and 22 c arein their cold state. The second crystal set 54 is aligned for frequencyconversion, i.e. harmonic generation, in the warm steady statecondition. That is, the crystals 22 aa, 22 bb and 22 cc are aligned suchthat the angles of the crystal structures of the crystals 22 aa, 22 bband 22 cc relative to the beam 16A are orientated for efficient harmonicgeneration when the crystals 22 aa, 22 bb and 22 cc are in their warmsteady state.

The alignment of the crystals 22 a, 22 b and 22 c of the first crystalset 52 and the alignment of the crystals 22 aa, 22 bb and 22 cc of thesecond crystal set 54 may be performed by physically adjusting thespatial orientation of the crystals 22 a, 22 b, 22 c, 22 aa, 22 bb and22 cc relative to the beams 16 and 16 a, respectively, and/or byadjusting the temperature of the crystals 22 a, 22 b 22 c, 22 aa, 22 bband 22 cc to alter the orientation of the crystal structures within thecrystals 22 a, 22 b, 22 c, 22 aa, 22 bb and 22 cc.

This alignment may be carried out by first running the laser 12 at aslow pulse rate, e.g. 1 Hz. The crystals 22 a, 22 b and 22 c of thefirst crystal set 52 are then aligned (as previously hereinbeforedescribed) to obtain optimum harmonic generation in their cold state.The output beam 16 a from the first crystal set 52 is monitored duringthis procedure to determine when the alignment of the crystals 22 a, 22b and 22 c is producing optimum harmonic generation, i.e. the poweroutput of the beam 16 a output from the first crystal set 52 ismonitored. Alignment of the crystals 22 aa, 22 bb and 22 cc of thesecond crystal set 54 is carried out by running the laser 12 at a highpulse rate, e.g. over 200 Hz, for a period of about 3-5 minutes. Thiswill bring the temperature of the crystals 22 aa, 22 bb and 22 cc of thesecond crystal set 54 to their warm steady state condition temperature.The crystals 22 aa, 22 bb and 22 cc are then aligned (as previouslyhereinbefore described) to obtain optimum harmonic generation. Again,the power output of the beam 16 b output from the second crystal set 54is monitored to determine the alignment of the crystals 22 aa, 22 bb and22 cc that produces optimum harmonic generation in the beam 16 b for thewarm steady state.

When the apparatus 10A is first switched on, all of the crystals in thefirst and second crystal sets 52 and 54 are in their cold state.Further, since the crystals 22 a, 22 b and 22 c of the first crystal set52 are aligned for harmonic generation in the cold state, the crystals22 a, 22 b and 22 c will produce high energy of all the harmonics in thebeam 16 a. Passing the beam 16 a through the second crystal set 54 wouldsuggest that back conversion would result, i.e. the short wavelengths inthe beam 16 a would be converted back to longer wavelengths, which wouldreduce the overall efficiency of the frequency conversion. However,since the crystals 22 aa, 22 bb and 22 cc of the second crystal set 54are aligned for harmonic generation in their warm steady statecondition, there would be little or no back conversion of the beam 16 aby the second crystal set 54 when the apparatus 10A is first switchedon. The second set of crystals 22 aa, 22 bb and 22 cc are exposed to allthe harmonics and close to their final energy. Thus, the crystals 22 aa,22 bb and 22 cc in the second crystal set 54 will quickly reach theirwarm steady state condition temperature.

The frequency conversion efficiency of the crystals 22 a, 22 b and 22 cof the first crystal set 52 will decrease as the crystals warm up.However, this decrease in frequency conversion efficiency of the firstcrystal set 52 will be compensated for by the increasing frequencyconversion efficiency obtained from the crystals 22 aa, 22 bb and 22 ccof the second crystal set 54 as they warm up and reach their warm steadystate condition.

Appropriate selection of properties of the crystals in the first andsecond crystal sets 52 and 54 makes it possible to have the changes infrequency conversion efficiency of the first and second crystal sets 52and 54 cancel each other so that the final energy in the output beam 16b is stable. The property that is most readily adjusted to achieve thisoptimum stable energy state is the lengths of the crystals 22 a, 22 band 22 c and the crystals 22 aa, 22 bb and 22 cc in the first and secondcrystal sets 52 and 54, respectively. However, both the material ofwhich the crystals in the first and second crystal sets 52 and 54 aremade and the temperature of those crystals are also properties that canbe selected or adjusted to achieve the optimum stable energy state.

In the preceding embodiment, the crystals 22 a, 22 b and 22 c of thefirst crystal set 52 are aligned for efficient harmonic generation inthe cold state and the crystals 22 aa, 22 bb and 22 cc of the secondcrystal set 54 are aligned for efficient harmonic generation in the warmsteady state. However, this may be reversed. That is, the crystals 22 a,22 b and 22 c of the first crystal set 52 may be aligned for efficientharmonic generation in their warm steady state and the crystals 22 aa,22 bb and 22 cc may be aligned for efficient harmonic generation intheir cold steady state. This alternative will, however, result inreduced performance efficiency. This is because in the alternative, theoutput beam 16 from the solid state laser does not first pass throughcrystals that are aligned for efficient harmonic generation in the coldstate.

In FIG. 5 there is shown an embodiment of another solid state laserapparatus 10B in accordance with the present invention. Similarcomponents of the apparatus 10B and the apparatus 10A are denoted bysimilar reference numerals in FIG. 5. The apparatus 10B comprises asolid state laser 12 similar to that of the apparatus 10 and 10A. Theapparatus 10B further comprises a frequency conversion apparatus 60.

The frequency conversion apparatus 60 is similar to the frequencyconversion apparatus 50 of the embodiment shown in FIG. 4 except that abeam splitter 62 is provided between the first and second crystal sets52 and 54. The beam splitter 62 reflects the shortest wavelengthcomponent of the output beam 16 a from the first crystal set 52 and thisis identified as the beam 16 c in FIG. 5. The remainder of the beam 16 apasses through the beam splitter 62. The remainder of the beam 16 a thatpasses through the beam splitter 62 is identified by reference numeral16 d in FIG. 5. The beam 16 d is a mixture of wavelengths including IR,but not any of the shortest wavelength component as that has beenreflected by the beam splitter 62. In the embodiment shown in FIG. 5,where three crystals are used in the first and second crystal sets 52and 54, the frequency conversion by the first and second crystal sets 52and 54 produces the fifth harmonic. Thus, the beam 16 c consists of thefifth harmonic wavelengths. The beam 16 d passes through the crystals 22aa, 22 bb and 22 cc of the second crystal set and is able to undergofrequency conversion to produce the output beam 16 e. The output beams16 e has the same wavelength as the beam 16 c.

The beams 16 b and 16 c may be directed to a target point 64 byrespective mirrors 66 and 68.

Alternatively, the beams 16 b and 16 c may be directed to separatetarget points (not shown) in which case the arrangements of the mirrors66 and 68 are omitted.

In FIG. 6 there is shown an embodiment of a laser apparatus 10C inaccordance with the present invention. Similar components of theapparatus 10C and the apparatus 10B are denoted by similar referencenumerals in FIG. 6. The apparatus 10C comprises a solid state laser 12similar to that of the apparatus 10B. The apparatus 10C furthercomprises a frequency conversion apparatus 70.

The frequency conversion apparatus 70 comprises first and second crystalsets 52 and 54, beam splitter 62, mirrors 66 and 68 similar to those ofthe apparatus 10B. The frequency conversion apparatus 70 furthercomprises polarisation optics 72 to combine the beams 16 c and 16 e(which are the output beams resulting from frequency conversion by thefirst and second crystal sets 52 and 54, respectively). The polarisationoptics 72 combine the two beams 16 c and 16 e to produce a single outputbeam 16 f.

The frequency conversion apparatus 14 hereinbefore described withreference to FIGS. 1 a to 3 and a frequency conversion apparatus 10A,10B or 10C hereinbefore described with reference to FIGS. 4, 5 and 6,respectively, may be incorporated in a single solid state laserapparatus.

By way of example of this, FIG. 7 shows the frequency conversionapparatus 14 and the frequency conversion apparatus 70 incorporated in asingle solid state laser apparatus 10D.

In the solid state laser apparatus 10D, the single crystal 22 of thefrequency conversion apparatus 14 incorporated in the solid state laserapparatus 10 is replaced by first and second crystal sets 52 and 54 asused in the frequency conversion apparatus 70 (shown in FIG. 6), theoptic 18 is provided prior to the first crystal set 54 and the optic 20is provided after the polarisation optics 72.

The beam 16 emitted by the solid state laser device 12 is deviated andcaused to spatially move, i.e. be scanned, following passagetherethrough as previously hereinbefore described with reference to thesolid state laser apparatus 10. This results in the beam being scannedwhen it passes through or is reflected by the other components in thesolid state apparatus 10D, i.e. from the first crystal set 52 to theoptic 20. The optic 20 de-scans the beam as it passes therethroughresulting in a de-scanned beam 16 g. In FIG. 7, a superscript (‘) isused to denote the beam when it is in a scanned condition.

In an analogous manner, the frequency conversion apparatus 14 of thesolid state laser apparatus 10 can be incorporated in a single solidstate laser apparatus with the frequency conversion apparatus 50, shownin FIG. 4, or the frequency conversion apparatus 60, shown in FIG. 5.

In any such arrangements in which the frequency conversion apparatus ofthe solid state laser apparatus 14 shown in FIGS. 1 a to 3 isincorporated in a single solid state laser apparatus with one of thefrequency conversion apparatuses 50, 60 or 70, the second optic 20 maybe omitted. Omission of the optic 20 has been previously hereinbeforedescribed with reference to the embodiment illustrated in FIGS. 1 a to3. Omission of the optic 20 means that the final beam is not de-scanned.

It is also to be understood that the embodiment described with referenceto FIGS. 5, 6 and 7 and variations thereof as hereinbefore described mayuse a lesser or greater number of crystals 22 in the first and secondcrystal sets 52 and 54. However, in normal operation, the same number ofcrystals 22 would be used in the first and second crystal sets 52 and54.

A Brewster window may be provided at a suitable location on the beamoutput side of the frequency conversion systems hereinbefore described.

Modifications and variations such as would be apparent to the skilledaddressee are deemed to be within the scope of the present invention.

1. A frequency conversion apparatus comprising: a first crystal sethaving at least one non-linear optical crystal, a second crystal sethaving at least one non-linear optical crystal, said first crystal setis arranged to receive a beam of coherent radiation that passes throughsaid first crystal set, said second crystal set is arranged to receivesaid beam after passage through said first crystal set, and said beampasses through said second crystal set, wherein one of said first andsecond crystal sets is aligned for efficient harmonic generation byfrequency conversion of said beam in a cold state of that crystal setwherein said cold state is a state prior to heating of a crystal set byabsorbed laser energy and the other of said first and second crystalsets is aligned for efficient harmonic generation by frequencyconversion of said beam in a warm steady state of said other crystalset, wherein said warm steady state is a state when a crystal set hasbeen heated by absorbed laser energy and the heating has reached asteady state.
 2. A frequency conversion apparatus according to claim 1,wherein the first and second crystal sets are aligned for efficientharmonic generation by frequency conversion for said cold state or saidwarm steady state by adjusting the physical orientation of a respectivesaid crystal in said first and second crystal sets.
 3. A frequencyconversion apparatus according to claim 1, wherein the first and secondcrystal sets are alignable for efficient harmonic generation byfrequency conversion in said cold state or said warm steady state byaltering the temperature of a respective said crystal in said first andsecond crystal sets to adjust the crystal structure of said respectivecrystal.
 4. A frequency conversion apparatus according to claim 1,wherein said first crystal set is aligned for efficient harmonicgeneration by frequency conversion in the cold state and said secondcrystal set is aligned for efficient harmonic generation by frequencyconversion in the warm steady state.
 5. A frequency conversion apparatusaccording to claim 1, further comprising beam splitter means to divert acomponent beam of shortest wavelength of the output beam from said firstcrystal set away from the remainder of the output beam from said firstcrystal set and said beam splitter means allows the remainder of theoutput beam from said first crystal set to pass to said second crystalset for efficient harmonic generation by frequency conversion therein.6. A frequency conversion apparatus according to claim 5, furthercomprising beam combining means to receive the component beam ofshortest wavelength diverted by said beam splitter means and the outputbeam from said second crystal set and to combine them into a singlebeam.
 7. A frequency conversion apparatus according to claim 6, whereinsaid beam combining means comprises polarization optics.
 8. A laserapparatus comprising: a solid state laser, and a frequency conversionapparatus according to claim
 1. 9. A method of improving the frequencyconversion efficiency of a frequency conversion system, of a laserapparatus, that has a first crystal set having at least one non-linearoptical crystal, the method comprises: placing a second crystal sethaving at least one non-linear optical crystal at the beam output end ofsaid first crystal set, aligning one of said first and second crystalsets for efficient harmonic generation by frequency conversion of aninput beam in the cold state of that crystal set, and aligning the otherof said first and second crystal sets for efficient harmonic generationby frequency conversion of an input beam in the warm steady state ofsaid other crystal set, wherein said cold state is a state prior toheating of a crystal set by absorbed laser energy and said warm steadystate is a state when a crystal set has been heated by absorbed laserenergy and the heating has reached a steady state.
 10. A methodaccording to claim 9, wherein aligning of one said first and secondcrystals sets for efficient harmonic generation by frequency conversionof said beam in the cold state or the warm steady state comprisesadjusting the physical orientation of the respective crystal in saidfirst and second crystal sets.
 11. A method according to claim 9,wherein aligning said first and second crystal sets for efficientharmonic generation by frequency conversion in the cold state or thewarm steady state comprises altering the temperature of the respectivesaid crystal in said first and second crystal sets to adjust the crystalstructure of said respective crystal.
 12. A laser apparatus comprising:a solid state laser, and a frequency conversion apparatus according toclaim
 2. 13. A laser apparatus comprising: a solid state laser, and afrequency conversion apparatus according to claim
 3. 14. A laserapparatus comprising: a solid state laser, and a frequency conversionapparatus according to claim 4.